1. Field of the Invention
The present invention relates to a flow rate detecting device, which has a heating element and is used for measuring the flow velocity or flow rate of a fluid according to a heat transfer phenomenon where a heat is transferred to the fluid from the heating element or from a part heated by the heating element, and to a flow rate sensor employing such a flow rate detecting device. The present invention is applied to, for example, a flow rate sensor for use in measuring an intake air amount of an internal combustion engine.
2. Description of the Related Art
FIGS. 15 and 16 are a plan view and a sectional side view of a conventional flow rate detecting device described in, for example, Japanese Unexamined Patent Publication No. 4-230808 Official Gazette, respectively. Incidentally, the Japanese Unexamined Patent Publication No. 4-230808 Official Gazette describes the flow rate detecting device as a diaphragm sensor.
In FIGS. 15 and 16, reference numeral 101 designates a plate-like substrate constituted by a silicon semiconductor. In the central part of the rear surface portion of this plate-like substrate 101, a cavity 110 having a trapezoidal section is formed by, for instance, anisotropic etching in such a manner as not to reach the surface thereof. In the front surface portion of the plate-like substrate 101, a thin-film-like flow rate detecting diaphragm 102 (hereunder referred to simply as a diaphragm) is integrally formed. Further, a thin film heating element 103 is formed in the central part of the front surface portion of this diaphragm 102. Thin film temperature detecting elements 104 and 105 are formed on both sides of the heating element 103 in such a way as to be apart from each other by a predetermined distance and to be placed symmetrically with respect to the heating element 103. Further, each of elongated holes 106a and 106b penetrating the diaphragm 102 are bored in a portion between the heating element 103 and a corresponding one of the temperature detecting elements 104 and 105 along the longitudinal direction thereof. Moreover, two lines of plural rectangular holes 107a and 107b penetrating the diaphragm 102 are bored along the longitudinal direction of the heating element 103 outside the temperature detecting elements 104 and 105, respectively. Similarly, holes 108c and 108d penetrating the diaphragm 102 are bored in portions provided on both sides in the longitudinal direction of the heating element 103. Furthermore, holes 109c and 109d penetrating the diaphragm are bored in portions provided on both sides in the longitudinal direction of each of the temperature detecting elements 104 and 105. These holes are formed by using photolithography and wet etching (or dry etching) techniques.
When an energizing current for the heating element 103 is controlled by using such a conventional flow rate detecting device so that the temperature of the heating element 103 is higher than a to-be-measured fluid by a predetermined value, the temperatures of the temperature detecting elements 104 and 105 are equal to each other in the case that no movement of the to-be-measured fluid occurs (that is, the flow velocity=0).
When an air flow moves in the direction of an arrow A, the temperature of the temperature detecting element 104 positioned at the upstream-side place is lower than that thereof in the case that the flow velocity=0. As the flow velocity increases, the temperature thereof falls. Conversely, the temperature of the temperature detecting element 105 positioned at the downstream-side place does not lower to the temperature exhibited by the upstream-side temperature detecting element 104 at the same flow velocity. Thus, if a quantity corresponding to the difference in temperature between the temperature detecting elements 104 and 105 is obtained by incorporating the temperature detecting elements 104 and 105 into a Wheatstone bridge circuit (not shown), the flow velocity of the to-be-measured fluid can be measured.
The Japanese Unexamined Patent Publication No. 4-230808 Official Gazette describes the following advantages of the conventional flow rate detecting device owing to provisions of holes in the diaphragm 102. Namely, variation in output thereof due to the deposition of dusts is decreased as a result of the facts that heat flow from the heating element 103 to the temperature detecting elements 104 and 105 is decreased and that thus, the temperatures of the temperature detecting elements 104 and 105 are lowered. Moreover, the sensitivity of the conventional flow rate detecting device is enhanced because heat generated in the heating element 103 and transmitted to the plate-like substrate 101 is reduced.
Meanwhile, when the flow velocity of the fluid to be measured increases, or when pressure is applied to the flow rate detecting diaphragm 102, or when the flow rate detecting diaphragm 102 undergoes vibrations of large amplitudes, stress occurs in the diaphragm 102. At worst, the diaphragm 102 breaks.
Further, the conventional flow rate detecting device is constructed so that the holes are positioned in the vicinity of the upstream side of the heating element 103 and the temperature detecting elements 104 and 105. Thus, in the case of the long-term use of the flow rate detecting device, dusts contained in the fluid to be measured are deposited on the surface in the direction of thickness of the substrate at end portions of the holes. This results in change of the manner of flow of the fluid to be measured. Consequently, the conventional flow rate detecting device has a drawback in that the detection characteristics thereof vary.
Even if the holes bored in the diaphragm 102 are eliminated so as to prevent variation in the detection characteristics, the diaphragm 102 sometimes has insufficient strength.
When such a flow rate detecting device is employed as, for example, an intake air amount sensor to be used for controlling fuel supply to an internal combustion engine of an automobile, the following problems occur. The internal combustion engine of an automobile generates vibrations corresponding to 40 G to 50 G of force. Further, the flow velocity of the intake air sometimes reaches 200 m/s or more. Furthermore, when a back fire occurs, a pressure close to 2 atms may be applied to the flow rate detecting device. When the conventional flow rate detecting device is subject to such mechanical stress, this device is in danger of being broken.
Conversely, if the thickness of the diaphragm is increased to enhance the strength thereof, the heat capacity of the diaphragm increases. Moreover, the heat responsibility of the diaphragm is decreased. Consequently, the conventional flow rate detecting device cannot follow the variation in the flow rate.